1. Technical Field
The present invention relates to an electrode substrate for detecting a subject material in various fields and a detection device equipped with the substrate. More particularly, the invention relates to an electrode substrate that detects an exchange of carriers such as electron between the subject material and the electrode substrate, and relates to the detection device and the like equipped with the substrate.
2. Related Art
Since the mapping of the human genome has finished, a detection device that can efficiently and precisely identify biomolecules such as deoxyribonucleic acid (DNA), protein and antibody molecules has been playing an important role. The detection device can detect the information about the structure, function, weight, electric property and optical property of the sample containing the biomolecules and can transmit the information as data. As such detection device, for example, there is a biochip that can analyze a mass of samples in a short period of time. U.S. Pat. No. 5,445,934 is a first example of related art, and U.S. Pat. No. 6,280,590 is a second example of related art. The first example describes that the biochip adopts a method to measure fluorescence intensity for detecting DNA hybridization. The second example describes that the biochip adopts a method to measure a difference in DNA displacement that varies depending on the applied-electric field. Monitoring the intensity variation of the fluorescent reaction is becoming a mainstream method in this field as described in the examples.
Moreover, there has been an increasing demand for a sensor or a microchip that can detect biological reactions related to the biomolecules such as enzyme, DNA and antibody in real time and in vitro rather than in vivo. After the human genome project, a function analysis of DNA strand has becoming the mainstream of the study. Especially, the function analysis of the proteins including the enzyme composed of the DNA strand and the antibody, and optimization of a target used in the drug discovery according to the result of the analysis are becoming a mainstream trend. In order to efficiently conduct the analysis, in other words, for a high throughput, it is important to utilize a DNA chip and a protein chip. The key of the chip technology is a capability of a biointerface (hereinafter called “BI”) that serves as a detection mechanism between the reaction of the biomolecules and a detection method (light detection such as fluorescence, electrochemical detection, detection of small weight and the like).
The BI needs to be capable of sorting out the useful information of the biological reactions, amplifying the useful information parameter, and converting or transferring the information parameter to a detection parameter.
Following the practical application of the electrochemical detection device utilizing an enzyme molecule which is a representative example of the detection device having the BI function, a great demand for such detection devices is expected in the future. To be more specific, there is a detection device for monitoring blood sugar levels of diabetic patients. JP-A-6-78791, JP-A-6-90754 and JP-T-8-78791 are third-fifth examples of related art. As described in the third-fifth examples, enzyme molecules of glucose-oxidase or glucose-dehydrogenase that oxidizes glucose molecule to be gluconic acid are immobilized on an electrode substrate. Glucose contained in the blood is oxidized so as to be the gluconic acid in an enzyme molecular film on the substrate, generating an oxidation current. Accordingly, the blood sugar levels can be measured in real time by detecting the generated oxidation current that is captured with the electrode.
Generally, in the detection device for monitoring the blood sugar levels, a solution in which the biomolecule such as enzyme molecule is dispersed in a water-soluble polymer such as cellulose is applied on the electrode by a spin-coat method and the like, and a mixed dispersion film is formed. Alternatively, the biomolecule can be immobilized or semi-immobilized (loose retention by noncovalent binding) on the surface of the electrode substrate by utilizing a self-assembled monolayer (hereinafter called “SAM”). The blood sugar levels can be monitored by detecting a pseudo-biological reaction occurred on the solid surface. This biomolecule immobilization method utilizing the SAM is rapidly becoming the mainstream in this field so far.
However, some problems are pointed out in the above-described method of immobilizing the SAM on the surface of the electrode substrate as follows: (1) It is difficult to control the interaction between the surface of the electrode substrate and the biomolecule since the SAM is a monolayer film. For example, if the biomolecule contacts with a metal surface, the biomolecule, especially the enzyme and the like, could be denatured and the activity of the enzyme could be lost. (2) It is difficult to control the nonspecific adsorption between the surface of the electrode substrate and the biomolecule. For example, the biomolecule could be absorbed to the surface of the electrode substrate with an electrostatic force and van der Waals' force. (3) It is difficult to tell that the device is monitoring either the oxidation current generated in the enzyme reaction or a leakage current because the monolayer film is too thin. (4) With the hitherto known SAM, it is difficult to detect an oxidation-reduction current of the enzyme and the like generated in the electrode substrate under the film because the SAM has a high insulation quality. Otherwise, it is difficult to form a selectively permeable membrane because the film thickness and the density of the SAM are so small that the leakage current tends to be generated.